That is impossible without hacking into the game, and I strongly suggest you do not. Legendary Pokemon have ridiculous high stats and will just lessen the fun of the game. The normal starters can be stronger than legendary Pokemon if you train them and give them good moves, so why have a legendary? Besides legendaries are really slow in levelling and you will most likely end up underleveled.
you can start over
the legendary Pokemon in diamond and pearl are dialga and palkia.
No, a Bidoof cannot evolve into any Legendary Pokémon in Diamond.
Heatran
a lot of legendary Pokemon
you cant unfourtanately
Dialga
when you see the legendary Pokemon on the start screen press up select and b at the same time
arceus
you can't catch it because you already beaten the legendary
The video games Pokemon Diamond and Pokemon Pearl introduced two Legendary Pokemon whose names start with the letter M: Mesprit and Manaphy. Mesprit is naturally encountered in game, while Manaphy was only obtainable from a special event that required another game, Pokemon Ranger.
you can't unless you have AR